1. Field of the Invention
The present invention relates to a transmitter, more particularly to suppression of sidebands in allow intermediate frequency transmitter. Such a transmitter can be a transmit branch of a time division duplex transceiver that has a common local oscillator for the transmit branch and the receive branch of the transceiver, e.g, a transceiver operating in the so-called 2.4 GHz ISM band, or any other suitable transmitter or transceiver.
2. Description of the Related Art
In time division duplexing transceivers, in order to reduce cost, it is desirable to have a common local oscillator for the transmitter and the receiver. Furthermore, in order to achieve a short transmit-receive turn-around time, the same synthesizer based local oscillator frequency should be used for the transmitter and the receiver. Moreover, in order to be able to integrate receive channel filters, or to produce transmitter base band I and Q signals via digital-to-analog converters while applying a low sampling rate, the intermediate frequency should be kept low with respect to the radio frequency. Particularly when such a common-LO and low-IF transceiver operates at a microwave frequency range, e.g. at a 2.4 GHz ISM (Industrial Scientific and Medical) band, the transmitter exhibits carrier leakage, and, in addition to a desired transmit band within the ISM band, its sideband is-transmitted. In order mitigate DC offset related problems in the receiver, a low IF architecture is preferable. However, to keep the synthesizer frequency the same, the transmitter operates with the same low IF frequency. This produces an unwanted sideband in the transmitter, at a frequency distance of twice the intermediate frequency from the central frequency of the desired transmit band. For low-IF, it is difficult to filter-out the unwanted sideband, particularly if it falls outside the ISM band.
It is an object of the invention to provide a transmitter wherein filtering requirements of filtering a sideband of a desired transmit band in a given band are highly obviated.
It is another object of the invention to ensure that the sideband does not fall outside the given band.
In accordance with the invention, a method for transmitting a high frequency transmit signal is provided, said method comprising:
generating a low intermediate frequency quadrature signal;
up-converting said low intermediate frequency; quadrature signal to said high frequency transmit signal, a frequency spectrum of said high frequency transmit signal having a transmit band and a sideband corresponding to said transmit band, said transmit band being confined to a total transmit band outside of which severe sideband filtering requirements apply;
before said up-converting, providing that said side band falls at a side of said transmit band such that said sideband falls within said total-transmit band; and
transmitting said high frequency transmit signal.
Because the sideband now always falls within the total transmit band, filtering requirements thereof are greatly obviated. In the ISM-band, for instance, sideband suppression should typically be better than xe2x88x9247 dBm. Inside the total transmit band, typically, the sideband should only be 20 dB below the transmit band.
In a first embodiment, transmit band and sideband exchange is done at up-converting, by exchanging quadrature local oscillator signals.
In a second embodiment, transmit band and sideband exchange is done at a low intermediate frequency, by exchanging low intermediate frequency quadrature signals.
In both embodiments, upon exchanging a transmit band and its sideband, the frequency of the local oscillator signal is frequency shifted over twice the low intermediate frequency.
In the first embodiment, preferably, exchange of quadrature local oscillator signals is done by controlling a phase of a buffer, controlling the phase between a first phase and a second phase that differ 180xc2x0.